The invention relates to silicone rubber compositions. In particular, this invention relates to silicone rubber compositions useful as high voltage insulators (HVI""s).
High voltage insulators such as those used in power transmission lines, distribution stations or surge arrestors are typically made of porcelain or glass. Under ordinary working conditions, these high voltage insulators stand long use. A severe contaminate environment such as a seaside or an industrial district can cause insulators to degrade. High electrical stress can cause droplets of water to join together into a larger single drop known as a filament. Water filaments dissolve conductive pollution to form conductive paths that lower surface resistance. As the conductive paths form along the surface of an insulator, ohmic heating, caused by leakage current, causes a decrease in resistance and a corresponding increase in current. The heating can cause evaporation and subsequent drying to form a xe2x80x9cdry-bandxe2x80x9d. The electrical stress across this xe2x80x9cbandxe2x80x9d or gap can be the site for electrical discharge activity (corona, arcing, and/or partial discharges). The discharge activity leads to flashover.
xe2x80x9cHydrophobicityxe2x80x9d as used herein refers to a lack of affinity or a repulsion for water or failure to adsorb water. A hydrophobic surface has a low surface energy. Water resides on the surface as discrete droplets. In contrast, a hydrophilic (water-loving) surface has a high free surface energy thereby enabling water to cover the surface as a film. Periods of wetness in a contaminate environment can cause electrical discharge activity on an insulator surface that destroys hydrophobicity. If the hydrophobicity property is not recovered, more water accumulates, accelerating dry band arcing and discharge activity.
Hydrophobicity can be determined by measuring a contact angle (CA) between a substrate surface and a surface of pure water. Silicones without additives typically have a contact angle of about 114xc2x0. After corona treatment to destroy hydrophobicity, the contact angle is typically in a range between about 10xc2x0 C. and about 50xc2x0 C. Hydrophobicity recovery is measured after a 24-hour rest period. Recovered contact angle (RCA) conventionally, is the contact angle after the recovery period subsequent to corona treatment.
Both hydrophobicity and recovered contact angle are important properties in high voltage insulators, particularly when used in harsh environments. Thus, there is a need for silicone compositions with improved hydrophobicity properties.
The present invention relates to a particulate material that imparts an improved hydrophobicity and improved recovered contact angle property to a silicone composition. The silicone composition comprises (A) a silicone polymer and (B) an inorganic hydrophobicity imparting particulate (HIP) that imparts improved hydrophobicity properties.
In another aspect, the present invention relates to a silicone composition, comprising (a) a silicone polymer in a range between about 15% and about 50%, by weight of the total composition; (b) a hydrophobicity imparting particulate in a range between about 1% and about 50% by weight of the total composition that imparts a hydrophobicity property to the total composition; (c) an anti-tracking agent and a flame retardant in a range between about 20% and about 70% by weight of the total composition; (d) a coupling agent in a range between about 0.01% and about 1% by weight of the total composition; (e) a curing agent in a range between about 0.1% and about 5% by weight of the total composition; (f) an extending filler up to about 20% by weight of the total composition; and (g) at least one processing fluid in a range between about 0.1% and about 5% by weight of the total composition.
In another aspect, the present invention relates to a silicone composition comprising (A) a silicone polymer and (B) two or more hydrophobicity imparting particulates that impart a hydrophobicity property to the composition or at least one hydrophobicity imparting particulate that imparts a hydrophobicity property to the composition and a filler.
In another aspect, the present invention relates to an insulator comprising a housing portion. The housing portion comprises a cured product of (A) a silicone polymer and (B) an inorganic hydrophobicity imparting particulate.
In still another aspect, the present invention relates to an insulator comprising a housing portion which comprises a cured product of (A) a silicone polymer and (B) two or more hydrophobicity imparting particulates that impart a hydrophobicity property to the composition or a hydrophobicity imparting particulate that imparts a hydrophobicity property to the composition and a filler.
The present invention also relates to a method of imparting a hydrophobicity recovery property to a silicone polymer composition. In the method, a target hydrophobicity recovery property is determined for a silicone polymer composition. An inorganic hydrophobicity imparting particulate is selected to impart a hydrophobicity recovery property to a silicone polymer composition. The inorganic hydrophobicity imparting particulate is then added into the silicone polymer composition in an amount to impart the hydrophobicity recovery property.
In another aspect, the present invention relates to a method of improving a hydrophobicity property of a cured silicone polymer composition. In the method, a target hydrophobicity property is determined for a cured silicone polymer composition. An inorganic hydrophobicity imparting particulate is selected that imparts a hydrophobicity property to a silicone polymer composition. A blend of (A) a silicone polymer and (B) the selected inorganic filler is then compounded and cured by heating.